Pediatric overweight, defined as BMI >= 95th percentile for the same age and gender, is one of the most prevalent nutritional problems affecting children. Prevalence estimates show a substantial increase of childhood overweight in the U.S. from 4.6% in 1970 to 15.5% in 2000. The goal of this proposed research project is to describe changes in body composition at the cellular level and to estimate the energy costs associated with these changes in normal-weight and overweight children. The energy cost of growth has two components: 1) growth energy expenditure (GEE) for synthesizing body cell mass (BCM) and extracellular components, and 2) the energy stored (SE) in the cells, basically as fat and protein. The energy costs of 9.4 kcal/g for fat storage and 4.6 kcal/g for protein storage have been established for the SE component. However, the GEE component during growth has never been studied quantitatively. Although it is recognized that overweight children gain excessive weight, little is known about the difference in GEE between overweight and normal-weight children. This study is based on a conceptual model that GEE is equal to the difference between basal energy expenditure (BEE) and maintenance energy expenditure (MEE), i.e., GEE = BEE - MEE. BEE can be measured by indirect calorimetry, and MEE can be estimated by a recently established cellular level MEE model based on organ and tissue masses measured by MRI (Wang et al., Am J Clin Nutr 2005;81:799). We will compare GEE between overweight and normal-weight children, and explore the association between GEE and the gains of BCM and extracellular solids (ECS, e.g., bone). We will test two hypotheses: 1) the gains of BCM and ECS are major contributors of GEE; and 2) overweight children spend a different amount of energy to construct BCM than normal-weight children of same age and gender. If the hypotheses are correct, a multiple regression analysis model including gains of BCM and ECS as independent variables should explain a large fraction of the variance in GEE. Accordingly, we could fully explain the well-known phenomenon that children have a higher BEE/FFM ratio (-64 kcal/kg per day for a 0.5 yr-old child) than do adults (-30 kcal/kg per day for adult man). Therefore, this study has the potential to answer an important question regarding the energy cost for synthesizing new BCM, and to contribute to our understanding of energy metabolism in growing normal-weight and overweight children. [unreadable] [unreadable] [unreadable]